Manufacture of magnetic alloys



Inc 27, 1933- v w. 5. SMITH ET AL 1,915,766

. MANUFACTURE OF MAGNETIC ALLOYS Filed Oct. 21. 1931 I Rtawrmn THINK/V555 @atented June 27, 1933 UNITED STATES PATENT oFF ca WILLOUGHBY STATHAM SMITH, OF NEWTON POPPLEFORD, HENRY JOSEPH GABNET'I, OF SEVENOAKS, AND WALTER FREDERICK RANDALL, F EWELL, ENGLAND MANUFACTURE or MAGNETIC ALLOYS Application filed October 21, 1931, Serial No. 570,252, ana m This invention relates to the manufacture of magnetic materials and alloys and more particularly to a method for increasing high magnetic permeability therein.

have found,

netizing field steeply than in a material worked either more can indeed be Variations in the magnetic properties of certain ferro-magnetic metals and alloys, especiall y at low magnetizing fields, have never been thoroughly explored or controlled, although subjected to much examination in later years. Differences in chemical composition, the presence or absence of impurities and dissolved gases, variations in heat treatment, grain size, and'internalstrains have all been considered as contributory causes. We

however, that ous factors properties, an important factor which materially influences the magnetic properties 0 the alloy, is the crystallomorphic orientation of the crystals of which the alloy is built up.

It is a known fact that in the case of a single crystal of iron, the magnetic permeability in a, direction at right angles to a cubeface rises more steeply with than in a. direction at right angles to a face of the rhombic dodecahedron, where, however, the permeability rises more direction at ri ht angles to an octahedron face. I On the ot ier hand, in the case of a single crystal of nickel, the magnetic permeability in the direction at right angles to an octahedron face rises more steeply than in the direction at right angles to a cube face, with the direction at right angles to a facet of the rhombic dodecahedron occupying an intermediate position.

We have found that ferromagnetic material in the form ofstrip upon which a mean amount of cold-work has been carried out possesses, after standard heat treatment, a higher permeability orless and subjected to the same'heat-treatment. We have also found that this increased permeability developed in the strip which has been subjected to the greater amount 0 work if the latter is subjected to a very niuc longer heat treatment than is that upon which the proper amount of cold work has been car ried out, whereas the lightly work strip, such while these vari-' have their efiect upon magnetic,

increasing magform if the heat treatment than a strip of the same Great Britain October 81, 1980.

a strip that has received hot-rolling but little or no cold rolling, will never fully respond to heat treatment. i

Our investigations have revealed that these phenomena are due to the presence or absence in the ferromagnetic materialof a particular form of crystallization. It seems that; high magnetic permeability is associated it definite crystal 1 orientation, which =1 the latter to the proper amount ofcold -iworf before t hea -treatment to develop the m p,

co d

of the strip,

been cold worked to a point where the crystal outline is substantially completely obliterated without, however, rendering the material amorphous, the cold working being arrested when the crystallographic axes are inclined to the plane of rolling recrystallization the octahedral face coincides with the rolled surface of the alloy. If the material has been cold worked to an excessive degree there is obtained after heat treatment a more or less unstable crystal orientation which, however, passes into the more stable is greatly prolonged.

The object of this invention, therefore, is so to precondition a piece of ferromagnetic material, which is composed not of a single crystal but of innumerable minute crystals.

that upon subjecting it to heat treatmentto develop magnetic permeability it will crystallize wholly or predominantly in the form in which the highest magnetic properties are manifested.

In accordance with the present invention. therefore, the amount of cold working of a magnetic material is controlled with the object of obtaining, after a final heat treatment an increased or higher magnetic permeability. 1

In one methodof carrying out the invention the ferromagnetic material in strip or sheet form isreduced by the ordinary known methods, such as by hot rolling h a r a meet I readily nduced in the matei" a1 ,by s ubj' tin in general the desired crystal orientation is produced when the material has followed by so that after cold rolling with the necessary intermediate condition for the development after heat the invention, and

treatment, of the highest magnetic permeabilit In practice the metal or alloy is melted by known methods with the usual precautions to obtain purity and constancy of composition. The metal is cast as an ingot in chilled moulds and reduced by hot rolling at a temperature of about 1000-1100 C. to a thickness between three and four finished strip required. The hot rolled strip is then reduced, without further annealing, by cold rolling to the finished size, the amount of cold rolling being suficient to develop on recrystallization the crystal form in which the metal or alloy exhibits its optimum magnetic properties.

It will be appreciated that the amount of cold working to which an alloy may safely be subjected, varies with the constituents of the alloy, and is dependent upon the. ultimate size or thickness required which also controls the amount of reduction that can be effected at each passage of the material throughthe rolling mills.

The invention is illustrated in the accompanying drawing in which Figure 1 is a diagram illustrating the variation in maximum permeability of a typical nickel iron alloy,

which is obtained after the necessary heat treatment by controlling the amount of cold rolling prior to heat treatment. Figures 2, 3 and 4' are photo micrographic views (magnification as 150)'of polished sections of an alloy showing the same respectively in underworked, mean worked in accordance with over worked conditions respectively. Referring to Figure 1, the y-co-ordinates denote permeability and the m-co-ordinates the percentage of final reduction in thickness of the alloy by cold rolling, this being the ratio that the thickness of the finished strip bears to the thickness of the strip following the last anneal. It will be noted that there is a peak value at about a 62% final reduction, but that high permeability values are obtainable between about 58-85% reduction.

some cases therefore, when a very thin finished size is required, it may be more economical to carry out two sta es of cold rolling'; for instance, in the pro notion of .005"

strlp, the ingot should be hot rolled to .06", cold rolled to .018-.02", then annealed to resoften the material, and finally reduced by further cold rolling to the thickness re- After the amount of cold working which is required in accordance with, this invention,

times that of the it has been found that for the alloy is subjected to the usual heat treatment for developing magnetic permeability.

Referring now to Figures 24, three samples were prepared from a nickel iron alloy of high nickel content containing small proportions of copper and other elements, by

reducing specimens of the alloy, by the usual methods of hot rolling and cold rolling with the necessary intermediate anneals, as described above, and then subjecting the specimens to the different final reductions in the cold, stated in the table below. The micrographs were taken after the final cold work but before heat treatment.

R Extent Mlcro- After heat graph ggi Remarks treatment I n I4 Figure 2. 40% Under-workedcrystal grains 15,000 30,000

still a parent. Figure 3... 66% Worke in accordance with 16,000 43,000

the invention-grains broken up but no striations as in overworked sample. Figure 4 85% overworked-striated appear- 12, 000 25, 000

ance clue to overwork.

The values of the initial andmaximum permeability given in the above table were obtained after heat treatment of the cold worked specimens in the usual manner; the permeability values of the cold worked" alloy prior to heat treatment are very low in all three cases.

The values of cold, stated in the particular alloy used, but vary the composition of the alloy; illustrated in the typical for all the in question.

By way of example it may be mentioned that it has been found necessary to cold work the alloysdescribed and, claimed in the U. S. Patent No. 1,656,959,. Smith and Garnett, of January 24, 1928, in order to develop in them after heat treatment high magnetic permeability. Careful investigation has shown, however, that these alloys must be cold worked to a definite extent, within not very critical limits, in order to develop after heat treatment the highest magnetic permeability and the final reduction in the of course with the structures micrograplis are, however, magnetic metals and alloys .005 'strip, the thicknesses given in the preceding paragraph are suitable limits for the different stages of reduction. The eifect'of heat treatingthis alloy strip preferably at about 1150 afterit has received the correct amount of cold work is to develop an octahedral structure in the direction of rolling which in these alloys is accompanied bv a very high magnetic permeability. If the cold working has been insufiicient, a cubic structure is developed, while another type of strulgture is developed after excessive cold wor When the alloy has been subjected'to exabove table, apply for the the production of I which is dependent upon the crystal formation required to give high magnetic perme ability.

By making small additions of other elements such as silicon, to the pure alloy, it is possible to render a form of crystallization, which the pure alloy assumes with ditiiculty, more easily attainable. The addition of such elements has the effect of causing the. alloy or metal to favour the form of crystallization desirable for high permeability, thereby making it much easier to control the crystallization by the preliminary cold work,

Example 1.A.n alloy containing:

' Per cent Iron 16. s Chromium 1. 6 Manganese 0. 7 Copper 5. 0 Silicon O. 02

and nickel the remainder, after reduction by described and annealvarying between mg gives values for 70,000 and 120,000.

[ 1. mm pie 2.An alloy containing:

Per cent Iron 16. 8 lhromiumsflu 1. 6 Manganese 0. 6 Copper 5. 0 Silicon 0. 20 and the remainder nickel, gives, when subjected to the optimum treatment, a value of ,-..m= 100,000 within r 10%. E mam/p10 3.An alloy having the composition:

Per cent N i 76.51 Fe 16. 05 Cu 4:. 90 Cr 1. 30 Mn 0. 40 Si -Q u 0. 18

was reduced to strip 0.015 inch in thickness in the. way described above, the final reduction by cold rolling being about The strip was then sheared to a width of 1; inch wound into a spiral of 3 inches internal diameter and 4.9 inches outside diameter, heat treated in a neutral atmosphere at 1120 for 30 minutes, and cooled in the annealing boa withdrawn from the furnace.

The permeability of H=0.005 gauss was same alloy when treated in the normal manner by the best methods available prior to this invention will develop an initial permeability of 7000 which is of the order 0 that of the ing magnetic this. sample at found to be 23,000. The

f to the finished size, example alloys of Patent No. erred to above, with which the present example alloy bears a close resemblance.

While, as a criterion of most alloys treated in accordance with the present invention, it

may be said that the initial permeability developed as a result of the mechanical working and heat treating is appreciably greater than that of iron (,1 about 250) as in the case of the nickel iron alloys having a high nickel content as described above, this may not i the case with all magnetic materials to which the invention may be usefully applied.

Thus for many applications resistance of the material is the leading factor. Generally speaking extremely hi 'h specific resistance can only'be obtained by sacrificing initial permeability, and in suc cases it may well bethat t 1e initial permeability before or even after heat treatment in accordance with the invention is less than that of iron, accordance withthe invention the permeability will be improved.

It will be ap reciated that this invention enables the prcgrred grain orientation to be developed in magnetic materials by graduating the amount of work upon the metal during reduction by cold rolling, and finally heat treating the rolled strip.

' The method of this invention for developmaterials by obtaining a preferred or entation of the grains by means of controlled cold work is tobe distin uished from the well known method of (ibtaining an increased grain'size b critical deformation, which method invo ves critically cold working the metal one or more times, an proposed for the treatment 0t iron and more articularly its silicon alloys in order to evelop their electrical properties for making transformer laminations.

What .we claim is l. A method of increasing magnetic meability in ferro-magnetic alloys, w ich comprises reducing the alloy by hot rolling toa thickness about three to four times the finished size, annealing, reducing by cold rolling to the finished size, and finally heat treating to develop the permeability.

2. A method of increasing magnetic permeability in nickel iron alloys which comprises reducing the alloy by hot rolling to a thickness about three to four times the fin- C. ished size, annealing, reducing, by cold rolling to the finished size, and finally heat treat in at 1150 to develop permeability.

.,A method of increasing magnetic rmeability in a ferroma netic material, w ich comprises subjecting t c material to a re determined amount of cold working, $11 as by rolling, in order to reduce the material and which working is suflicient to obliterate substantially completely the cry I outline without renderthe specific but as a result of treatment in i permeability in term-magnetic d which has been Y ing the material amorphous, and finally heat treating the material to develop permeability.

4. A method of increasing magnetic permeability in a ferromagnetic material, which comprises subjecting the material to a predetermined amount of cold working, such as by rolling, in order to reduce the material to the finished size,

tially completely obliterated and arrested at the stage of crystal orientation when the crystallographic axes are inclined to the plane of rolling and finally heat treating the material to develop permeabilit i 5. A method ofincreasing magnetic permeability in a nickel iron alloy which comprises subjecting the alloy to a predetermined amount of cold Working, such as by rolling in order to reduce the alloy to the finished size the working being continued until the crystal outline is substantially completely obliterated and the crystallographic the working being conrtlnued until the crystal outline is substanamma grains have been orientated so recrystallization the octahedral axes of the that after face coincides with the rolled surface of the alloy, and finally heat treating the alloy to cause recrystallization and develop permeability;

6. A method of increasing magnetic permeability in ferromagnetic alloys, which comprises reducing the alloy by hot rolling to a thickness about three to four times the finished size, annealing, reducing by cold rolling tothe finished size to obliterate substantially completely the crystal outline without rendering the material amorphous, and finally heat treating to develop the permeability. J i

In testimony that we claim the foregoing as our invention we have signed our names this ninth day of October, 1931.

WILLOUGHBY STATHAM SMITH. I HENRY JOSEPH GARNETT.

WALTERFREDERICK RANDALL. 

